126 



OF THE STRUCTURAL ELEMENTS OF THE HUMAN BODY. 



merely a sort of hour-glass contraction, is at last inflected so far as to constitute 

 a complete partition between the two halves of the original cell; and these 

 henceforth become two independent cells, which may go through the same pro- 

 cess in their turn (Fig. 7, A D). The repetition of this operation may take 

 place either in the same or in the contrary direction, so as to produce four cells, 

 either linearly arranged (E, F, G), or clustered together (H) ; and this dupli- 

 cation may go on upon the same plan, until a large mass has been produced by 

 the subdivision of a single original cell. In ordinary Cartilage, 1 it is most com- 

 mon to see the cells forming clusters (Fig. 41) ; but in Cartilage which is being 

 prepared for ossification, we see long lines of cells, which have been obviously 

 produced by the first of these methods of multiplication (Figs. 65, 66). 



Fig. 7. 



A B C D 



Multiplication of Cartilage-cells by duplication: A, original cell; B, the same beginning to divide; c, the 

 same showing complete division of the nucleus ; D, the same with the halves of the nucleus separated, and the 

 cavity of the cell subdivided; E, continuation of the same process, with cleavage in contrary direction, to form 

 a cluster of four cells ; F, G, H, production of a longitudinal series of cells, by continuation of cleavage in the 

 same direction. 



105. Not unfrequently, however, the multiplication of cells takes place, not 

 so much by the subdivision of the pre-existing cell, as by the development of 

 new cells in its interior ; these appear to take their origin in the nucleus, which 

 subdivides into two or more portions, each of them drawing a portion of the con- 

 tents of the parent-cell towards itself, and becoming converted into a cell by the 

 development of a cell-wall around this ; and they gradually increase in dimen- 

 sions, until they come to occupy the entire cavity of the parent-cell, and may so 

 distend its wall, by their further enlargement, that it can no longer be distin- 

 guished. Of this method of cell-formation, also, we have examples in cartilage, 

 especially in its early stage of development, when its growth is rapid (Fig. 8) ; 

 but we there seldom see more than three or four cells thus generated within 

 a parent-cell at any one time. It is in structures of more rapid growth, such 



1 It is thought by Dr. Leidy, who has carefully studied this process in Cartilage (see 

 his valuable paper "On the Intimate Structure and History of Articular Cartilages," in 

 the "Amer. Journ. of Med. Sci.," April, 1849), that the direction of the subdivision is 

 determined by that in which there is least resistance to the extension of the group of cells ; 

 but such can scarcely be the case in regard to the embryonic mass, the cells of which, if 

 this were the sole determining influence, would continue to multiply on a uniform plan ; 

 instead of which, as soon as they have arrived at a certain degree of minuteness of sub- 

 division, a diversity of arrangement begins to show itself in the component parts of what 

 was previously a homogeneous assemblage. 



